SITE TO SITE IPSEC CONFIGURATION

VPN can be describe as client and server technology architecture by which we are transmitting packet bits or emails message over the internet in a real sense when describing vpn concepts you need to understand how message or email is transfer to the internet without being tempered or manipulated by other hackers or skilled IT.
Actually  when you are on the internet chatting with your friends who is another remote location  you are chatting within an encrypted tunnels  especially this happen in a business corporate environment where you can see manager and senior  when they are outside of job they communicate using secure tunnel  in sense of that the way they will be exchange informational it will be in cipher text and when a hacker try to temper within their connection the vpn connection will go down or collapse in seconds and try to regenerate another route of communication without these two guys knowing that the connection  was reset due to someone want to temper in.
When you are connecting within the vpn connection there is  a lifetime given for key exchange actually these used as security mechanism in order the key can not being left on  fly without any communication being transfer which is bidirectional so the key concept is that vpn usually are very sensitive because corporate most of the corporate are running with sensitive data which are highly expensive in terms of making transaction.